1. Field of the Invention
The present invention relates to a heat-transfer structure and a substrate processing apparatus, and more particularly to a heat-transfer structure disposed in an evacuated chamber of a substrate processing apparatus.
2. Description of the Related Art
In general, etching apparatuses have been known as substrate processing apparatuses which process substrates using plasma. The substrate processing apparatuses are provided with an evacuated chamber in which plasma is generated. In the chamber, a mounting stage on which a wafer employed as a substrate is to be mounted is disposed. The mounting stage is provided with a disk-shaped electrostatic chuck (ESC) disposed on top of the mounting stage, and a focus ring made of, for example, silicon disposed around the outer peripheral edge of the upper surface of the electrostatic chuck.
In performing etching on a wafer, the wafer is mounted on the electrostatic chuck, pressure in the chamber is then reduced, process gas, for example, mixed gas comprised of C4F8 gas, O2 gas, and Ar gas is introduced into the chamber, and radio-frequency electric current is supplied into the chamber to generate plasma from the mixed gas. The plasma is caused to converge on the wafer by the focus ring to perform etching on the wafer. As a result of etching, the temperature of the wafer increases, but the wafer is cooled by a cooling mechanism incorporated in the electrostatic chuck. During cooling, helium (He) gas with high heat transference is caused to flow toward the back side of the wafer from the upper surface of the electrostatic chuck so as to improve heat transference between the electrostatic chuck and the wafer, whereby the wafer can be cooled with efficiency.
On the other hand, since a boundary between the back side of the focus ring and the outer peripheral edge of the electrostatic chuck is the boundary at which solid objects are in contact with each other, the degree of adhesion between the focus ring and the electrostatic chuck is small, and hence minute gaps are formed in the boundary. In particular, since the interior of the chamber is evacuated during etching, these minute gaps form a vacuum heat insulating layer, causing deterioration of heat transference between the electrostatic chuck and the focus ring and making it impossible to cool the focus ring with efficiency like the wafer. As a result, the temperature of the focus ring becomes higher than that of the wafer.
If the temperature of the focus ring is high, the outer peripheral edge of the wafer is higher in temperature as compared with the inside of the outer peripheral edge of the wafer, and hence the over-surface etching uniformity of the wafer deteriorates.
Also, reaction products are produced during etching and adhere as polymer films to side walls of the chamber and the focus ring. The polymer films protect the focus ring and others from plasma and prevent wear of the focus ring and others, but as shown in a graph of FIG. 5, the film thickness of polymer decreases as the temperature of an object (such as the focus ring) to which polymer adheres increases. Thus, as described above, when the temperature of the focus ring is high, polymer as reaction products is less likely to adhere to the focus ring, and hence the focus ring is directly exposed to plasma, causing the focus ring to wear out quickly.
Further, since the focus ring cannot be cooled with efficiency due to the formed vacuum heat insulating layer, heat accumulates over time, and thus the focus ring cannot be kept at a constant temperature during etching of wafers in the same lot. For this reason, processing performance deteriorates, and for example, distribution of etch rate varies from wafer to wafer.
To cope with this, the applicant of the present invention has proposed a method of increasing the degree of adhesion between the focus ring and the electrostatic chuck so as to improve heat transference between the electrostatic chuck and the focus ring as a counter measure. Specifically, a heat-transfer medium made of a heat-resistant elastic member such as conductive silicon rubber is interposed between the focus ring and the electrostatic chuck (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2002-016126).
According to this method, the heat-transfer medium becomes deformed between the focus ring and the electrostatic chuck. This improves the degree of adhesion between the electrostatic chuck and the focus ring, causing improvement of heat transference between the electrostatic chuck and the focus ring.
In recent years, however, the required level of over-surface etching uniformity of wafers has been increasing, and also, the demand for long-life focus rings have been growing. As a result, the focus ring has been required to be kept at a temperature 225° C. or less during etching. On the other hand, since the conductive silicon rubber which is a heat-transfer member has low fluidity, it cannot fully fill minute gaps in the boundary between the focus ring and the electrostatic chuck, and as a result, even if the heat-transfer member is interposed between the electrostatic chuck and the focus ring, there is a limit to improvement of heat transference between the electrostatic chuck and the focus ring, and it is difficult to keep the focus ring at a temperature of 225° C. or less during etching.